Dock levelers are commonly used at loading docks for the purpose of bridging the gap between a vehicle parked adjacent the loading dock and the loading dock itself. Typically, the vehicle will back up into a parked position wherein the rear of the vehicle engages bumpers disposed on the face of the building and intended to protect both the building and the rear of the vehicle from impact or defacement. With the vehicle backed against the bumpers, a gap still exists between the loading dock and the truck. The dock leveler is intended to bridge this gap.
Typically, the loading dock will be formed with a pit within which framing for the dock leveler is housed. The leveler itself comprises a deck pivotally attached at its rear end relative to the loading dock, usually to the framing. The deck is movable between a stored or "cross-traffic" position wherein the deck is even with the warehouse or building floor on either side of the pit, and a range of operating positions to accommodate various vehicle elevations. The stored position is referred to as a "cross-traffic" position since traffic moving in the warehouse can move over the deck easily as it forms an extension of the surrounding floor. At the front end of the deck, adjacent the parked truck, is a lip pivotally connected to the front end of the deck for movement between a pendant, or stored position and an extended position wherein the lip bridges the gap between the deck and the bed of the parked vehicle. With the leveler in this bridging configuration, fork trucks or personnel can pass between the loading dock and the bed of the parked vehicle for the purpose of loading and unloading the vehicle. As the vehicle is loaded or unloaded, and as the fork truck passes on and off of the vehicle, differing weights are exerted on the vehicle's suspension. As a result, the vehicle will typically move up and down throughout the loading or unloading procedure. The pivotal connection of the deck of the dock leveler allows the leveler to track this up and down movement of the vehicle.
In operation, the deck is first raised from the cross-traffic position (with the lip pendant) to a raised preparatory position. The power to raise the deck is provided either by springs (in the case of a so-called "mechanical leveler") or by a hydraulic cylinder or other actuator disposed between the framing and the deck. In a mechanical leveler a "holddown" device normally holds the leveler down against the upward bias of the springs. The holddown may be released to raise the deck by pulling the unit's main pull chain. Once the deck reaches the preparatory position, the lip is extended from its pendant position to an extended position. Subsequent downward rotation of the deck places the lip on the bed of the vehicle so as to bridge the gap between the dock and the vehicle. In a hydraulically-powered leveler, or one powered up by a different actuator, gravity provides the force necessary to rotate the leveler downward, while a mechanical leveler requires the weight of dock personnel to "walk down" the leveler to a position wherein the lip rests on the bed of the vehicle. As the vehicle moves up and down during loading or unloading, the leveler pivots up and down to maintain proper contact with the vehicle.
Since dock levelers are capable of pivoting in this manner, they preferably also include some means for preventing uncontrolled free fall of the deck in the event that the vehicle departs while a fork truck or other load is still disposed on the deck. Departure of the vehicle with a load still on the deck is typically referred to as "premature" or "unscheduled" since proper safety procedures require that the deck be unloaded before a vehicle departs. If premature departure were to occur without any means of free fall protection, such premature departure of the vehicle would mean that the lip was no longer in contact with the vehicle, and thus that the heavily loaded deck was effectively unsupported, and it would thus pivot downwardly through its full range of motion until it engaged the pit below. Given that a typical operating range for dock levelers is from 10 inches above dock height to 10 inches below dock height, it would be possible for a fork truck disposed on the leveler in such a situation to fall as much as 20 inches. The violent contact between the falling deck and the pit, as well as the substantial pitch at which the deck would then be disposed, could lead to undesirable results, such as the fork truck falling off the deck or goods or personnel being damaged or injured.
Different types of levelers may include different types of free fall protection intended to minimize the distance through which the deck may free fall before such downward movement is arrested. In the case of hydraulic levelers, the deck is powered through its range of motion by means of a hydraulic cylinder disposed between the underside of the deck and the pit below. To protect against free fall, the hydraulic cylinder typically includes a velocity fuse. The velocity fuse is intended to lock the hydraulic cylinder against further movement in the event that the deck achieves a certain velocity. Accordingly, if free fall begins to occur and the deck reaches this velocity, the hydraulic cylinder will be locked, and prevent the deck from further downward movement. Mechanical and other types of levelers, on the other hand, typically include so-called safety legs to limit free fall distance. An example of one type of safety leg mechanism is shown in the prior art FIGS. 2 and 3. The safety leg SL depends from the bottom of the deck and is adapted to engage a pedestal P disposed in the pit. Contact between the end of the leg SL and the pedestal P will arrest downward movement of the deck indicated as D. Thus, if a vehicle prematurely departs with a load on the deck, the deck will only "free fall" a limited distance--until the legs SL engage pedestal P. The legs SL typically remain in a supporting orientation for this purpose. For situations where the bed of the truck is significantly lower than dock height, the legs may be retracted rearwardly by a retracting mechanism R to allow the deck to angle downwardly below dock without the safety leg engaging the first stop S1 on the pedestal. To protect against free fall with the deck in a below-dock configuration, a second stop S2 is provided on the pedestal. The safety leg L is biased by a biasing member B, in this case a spring, toward the vertical position shown in FIG. 2. Accordingly, if the leveler is initially disposed below dock with the legs retracted and then is raised above dock by virtue of weight being removed from the truck and the vehicle suspension raising the leveler, the safety leg SL will return to the vertical orientation shown in FIG. 2.
While the safety leg configuration, and other similar safety leg configurations, provide the advantageous function of preventing substantial free fall in the event of premature or unscheduled departure of the vehicle with a load on the deck, they are not without their own limitations. One such limitation to previous safety leg configurations is shown if FIG. 3. In the circumstance shown in FIG. 3, the leveler is in a position wherein the safety leg SL engages the stop S1 on the pedestal P, thus preventing further downward movement of the leveler. As the fork truck moved onto the vehicle bed, however, the weight of the fork truck pushed the vehicle down further. The lip L was able to track this downward movement of the vehicle, since engagement of the safety leg SL with the stop S1 does not limit rotational movement of the lip L. The deck D, however, was prevented from moving to a lower position. The steep angle of the lip L may prevent the fork truck from being able to drive back up that slope and onto the deck D, or may at least cause a jarring collision between the lip and the fork truck. In the former circumstance, the fork truck may get trapped on the vehicle. This condition, typically referred to in the industry as "stump out" is an inconvenience, and represents a potential safety hazard to the fork truck operator who does not notice the significant angle of the lip. In addition, damage to either the lip, the leveler or the fork truck may occur as the fork truck attempts to drive back up the inclined lip. As the function of safety legs is otherwise very desirable, it would be advantageous to be able to provide a safety leg system that does not suffer from the disadvantages of stump out.
At least two attempts have been made in the prior art to address this issue. Both U.S. Pat. Nos. 3,995,342 and 5,440,772 include sensors that engage the bed of the vehicle along with the lip. In both cases, the sensor is shorter than the lip such that, in the event the vehicle prematurely departs, the sensor loses contact with the bed of the vehicle before the lip loses contact with the vehicle. When the sensor is in contact with the vehicle, the safety legs are retracted such that they are in a nonsupporting position. Upon the sensor losing contact with the bed of the vehicle, the safety legs are restored to a supporting position such that they would arrest downward movement of the leveler. Thus, when the vehicle departs, the sensor first loses contact with the vehicle moving the legs to a supporting position, and then the lip loses contact with the vehicle. This loss of contact between the lip and vehicle, however, does not result in substantial or uncontrolled free fall, since the legs have been restored to a supporting position. While these systems theoretically address the stump out problem, neither system proved to be workable in practice. For example, the system of the '342 patent includes a feature wherein the lip is latched into its extended position, and only could be unlatched by contact with the vehicle bed. Such a feature is problematic and potentially dangerous in the situation where the deck is raised and the lip is latched out, and then the deck is walked down without ever engaging a vehicle. As a result, the latched-out lip presents an obstacle and potential point of damage for a vehicle that backs into the loading dock while it is still latched in position. Another danger is that, in this scenario, if a fork truck were to drive onto a leveler with a latched-out lip, the leveler would rapidly rotate downward since the safety legs would not be in a supporting position. Moreover, both of these prior art systems included several pivot points, for the lip, the sensor, and the legs, and fairly complex mechanisms between these three members for the purpose of providing the desired safety leg function without stump out. The tolerances required to achieve proper functionality were difficult to achieve, leading to inconsistent function, as well as difficulty in manufacturability of these systems. The complex nature of the actuating mechanisms also led to increased expense for these systems.